The present invention relates, in general, to fault detection within a circuit, and more specifically relates to testing the circuitry used in a 77 GHz pulse Doppler radar sensor.
Vehicular radar systems have been developed for the automotive industry that provide a useful feedback as to the location and velocity of potential hazards. One such hazard discrimination system commonly found today uses a 77 GHz pulse Doppler radar sensor to identify and locate potential road hazards. Information from the radar sensor can be used to create a smart cruise control system that can automatically adjust to the conditions or hazards identified by the radar sensor.
Systems such as these utilize a small radar sensor module, usually about 4xe2x80x3 in diameter, that is mounted somewhere on the front of the host automobile (e.g., behind the front grill). The module contains one or more antennas for transmitting and receiving radar signals. These devices work by transmitting short pulses (approximately 36 nanoseconds) of 77 GHz energy. The pulse is then reflected back from any objects in its path. By measuring the time from transmission until the reflected wave is received, the distance and velocity of the object (in this case, another vehicle on the road) from the sensor can be calculated.
This technique is useful for determining only the distance and velocity of the object relative to the sensor. However, in order for the system to work properly, it is necessary to determine not only the distance to the object and velocity of the object, but also the radial location of the object relative to the vehicle. For example, it is important to know if the other vehicle is directly in front of the automobile, or if it is in a lane to the left or a lane to the right of the vehicle. In order to make such a determination, the radar sensor uses a plurality of beams, radiating at different horizontal angles, from the same antenna. Each beam is selected sequentially for the transmission and reception of pulse energy. By comparing the time difference from each of the reflected signals, the location of the object relative to the sensor can be calculated by using digital signal processing techniques. The sensor commonly used in such applications contains a series of three antenna switches to select the separate antenna feeds that form the three beams. Each antenna beam has a width of approximately 3 degrees. It has been determined that this beam width corresponds with the approximate width of a lane of traffic. Thus, by using the three separate signals, it is possible to check for the presence of vehicles in the same lane as the host vehicle or in one lane to either side of the host vehicle.
While pulse Doppler radar systems such as the one described above are useful in determining the location of other vehicles on the road, it is difficult to determine when a problem exists in the system that prevents it from operating. The output of a non-functioning system could be mistaken for an indication that no vehicles are present. Thus, it is important to know if the radar detection system can be relied upon.
In the prior art, system diagnostics were performed by using various detector circuits added to the 77 GHz pulse Doppler radar circuit. The only components of the system that were normally monitored was the input oscillator. This was accomplished by measuring the current drawn by the 77 GHz InP Gunn diode source. However, the 77 GHz InP Gunn source is no longer used in most 77 GHz pulse Doppler radar circuits as the input source. The 77 GHz InP source has been replaced with monolithic microwave integrated circuit chips (MMIC) that generate a 77 GHz input signal. As a result, the current monitoring technique used in the prior art can not be used in systems where the 77 GHz InP Gunn source has been replaced with MMIC technology.
An additional shortcoming of the existing fault detection process is that the current monitoring process can only detect a failure with respect to the InP Gunn 77 GHz input source. The current monitoring method will not detect a failure of any switches, mixers, or bond wire connections used in the 77 GHz pulse Doppler radar circuit. In order to test any of these components, additional detector circuits need to be incorporated into the system for each component that is to be monitored. Obviously, this increases the complexity of the circuit greatly, and as a result, also increases the cost of the system.
The present invention provides an improved fault detection system for a 77 GHz pulse Doppler radar sensor circuit. It accomplishes the fault detection process without the necessity of additional components such as detector diodes or detector circuits.
In a preferred embodiment, the present invention uses a novel configuration of the MMIC switch that is already present in the 77 GHz pulse Doppler radar sensor to perform the fault detection process. The MMIC switch is configured in a manner that allows for the testing of the input oscillator, mixer, bond connections, antenna connections and the switch itself.
The preferred embodiment of the present invention enables the 77 GHz pulse Doppler radar sensor to be tested by configuring the MMIC switch into two test mode configurations. Each configuration is used to test certain elements of the 77 GHz pulse Doppler radar sensor.
The first configuration tests all of the elements of the circuit with the exception of those associated with the antenna, namely the antenna switches and antenna bond wires. In this configuration, both the transmit (TX) and receive (RX) switches contained within the MMIC switch are closed. In addition, all of the switches to the antenna are open, so that none of the input signal is actually transmitted. This allows for the input signal from the 77 GHz oscillator to pass through the MMIC switch and reach both outputs of the MMIC switch at the same frequency with a phase difference of approximately 0 or 180 degrees. The two outputs from the MMIC switch become the inputs to a mixer. In this configuration, the mixer operates as an amplitude detector. By combining the two signals in the mixer, a DC output is achieved if the circuit is operating properly. If a DC output is not achieved at the mixer output, a fault has occurred within the circuit.
The second test mode configuration of the MMIC switch tests the antenna switches and associated bond wires. In this configuration, the RX switch is closed and the TX switch is opened. This reduces the amount of signal that reaches the antenna switches, but does not eliminate it entirely as a result of signal leakage across the TX switch. Each antenna switch is then individually modulated, while all other antenna switches in the circuit are opened. The outputs from the MMIC switch are provided to the mixer in a similar fashion to that of the first test mode configuration. The mixer output will show a modulation if the particular antenna switch being modulated is functioning properly. This process is repeated for each antenna switch to assure that they are all in working order.